332

Tuning the interactions between electron spins infullerene-based triad systems

Maria A. Lebedeva1, Thomas W. Chamberlain1, E. Stephen Davies1,Bradley E. Thomas1, Martin Schröder1 and Andrei N. Khlobystov*1,2

Full Research Paper Open Access

Address:1School of Chemistry, University of Nottingham, Nottingham, NG72RD, UK and 2Nottingham Nanoscience & Nanotechnology Centre,University of Nottingham, University Park, Nottingham, NG7 2RD, UK

Email:Andrei N. Khlobystov* - andrei.khlobystov@nottingham.ac.uk

* Corresponding author

Keywords:carbon nanomaterials; electrochemistry; EPR; fullerene dimers;fullerene triads; spin–spin interactions

Beilstein J. Org. Chem. 2014, 10, 332–343.doi:10.3762/bjoc.10.31

Received: 25 September 2013Accepted: 06 January 2014Published: 05 February 2014

This article is part of the Thematic Series "Functionalizedcarbon-nanomaterials".

Guest Editor: A. Krueger

© 2014 Lebedeva et al; licensee Beilstein-Institut.License and terms: see end of document.

AbstractA series of six fullerene–linker–fullerene triads have been prepared by the stepwise addition of the fullerene cages to bridging

moieties thus allowing the systematic variation of fullerene cage (C60 or C70) and linker (oxalate, acetate or terephthalate) and

enabling precise control over the inter-fullerene separation. The fullerene triads exhibit good solubility in common organic solvents,

have linear geometries and are diastereomerically pure. Cyclic voltammetric measurements demonstrate the excellent electron

accepting capacity of all triads, with up to 6 electrons taken up per molecule in the potential range between −2.3 and 0.2 V

(vs Fc+/Fc). No significant electronic interactions between fullerene cages are observed in the ground state indicating that the indi-

vidual properties of each C60 or C70 cage are retained within the triads. The electron–electron interactions in the electrochemically

generated dianions of these triads, with one electron per fullerene cage were studied by EPR spectroscopy. The nature of

electron–electron coupling observed at 77 K can be described as an equilibrium between doublet and triplet state biradicals which

depends on the inter-fullerene spacing. The shorter oxalate-bridged triads exhibit stronger spin–spin coupling with triplet character,

while in the longer terephthalate-bridged triads the intramolecular spin–spin coupling is significantly reduced.

332

IntroductionFabricating molecular systems that are capable of storing one or

more unpaired electrons is essential for the development of

molecular spintronics and electron-spin-based quantum

computing. Endohedral fullerenes are compounds that contain a

heteroatom trapped inside the fullerene cage and are able to

support the formation of stable radical materials [1]. They can

also show interesting properties such as magnetism, and

photoactivity and are thermally and chemically stable. Their

ability to form well-ordered 1D arrays makes them leading

candidate materials for the study of polyfunctional materials.

Beilstein J. Org. Chem. 2014, 10, 332–343.

333

For example, significant effort has been directed into incorpor-

ating N@C60 molecules into quantum computing devices [2].

Incorporating a second radical centre into these molecules, in

addition to the endohedral atom, introduces a mechanism to

control the magnetic properties of the resulting materials. This

is essential for the recording, storing and read-out processes

performed in spin-based quantum information processing using

nanoscale molecular architectures [3]. This has been achieved

recently within a copper porphyrin–N@C60 dyad [4] and

several types of N@C60–N@C60 molecules [5-7]. However the

application of these systems is limited due to a number of syn-

thetic challenges associated with the preparation and purifica-

tion of endohedral fullerenes [8]. This notwithstanding,

fullerene cages are excellent electron acceptors and can support

up to six electrons per fullerene cage to form species containing

one or more unpaired electrons, in which overall charge and the

spin state can be controlled precisely by applied potential [9]. In

addition, combining two fullerene cages within the same mole-

cule increases the total spin-carrying capacity and introduces a

mechanism of spin-tuning while retaining the intrinsic prop-

erties of each of the fullerene cages [10]. The synthesis of such

fullerene–bridge–fullerene triads, though not straightforward,

has been reported [11], the simplest involving species where the

fullerene cages are connected directly by a C–C bond [12], a

bridging O-atom [13], or by a transition metal atom [14]. A

variety of more complex triads have since evolved in which the

fullerene molecules are connected using optically or electro-

chemically active spacers [15]. The choice of linker in triad

systems is crucial as it has a significant impact on the prop-

erties of the resulting arrays [16]. As the strength of dipolar

coupling between unpaired electrons decreases as a function of

r−3, where r is the average distance between unpaired electrons,

the strength of any electron–electron interactions in fullerene

triads rapidly decreases with increasing distance between

fullerene cages [17]. Thus, the ability to control the inter-

fullerene separation is crucial in fabricating systems in which

specific interactions between multiple unpaired electrons are

targeted.

The shape of the fullerene containing molecule is also very

important. 1D and 2D ordering is a critical factor in the design

of molecular electronics. For example, linear molecules can be

ordered readily into 1D arrays using carbon nanotubes as

templates [18] and are therefore advantageous compared to non-

linear or branched molecules for which 1D packing arrange-

ments are inhibited. In addition, solubility can also be a signifi-

cant issue as fullerene triads tend to show poor solubility [19].

The majority of fullerene triads reported to date are either

synthesised via complicated non-scalable synthetic procedures,

which makes controlling the fullerene–fullerene distance diffi-

cult, or incorporate bulky spacers and solubilising groups

resulting in cumbersome non-linear structures and hence are not

ideal for potential applications in molecular electronics and

spintronics devices. We recently reported a new general syn-

thetic methodology for the formation of fullerene triads which

allows the introduction of fullerene cages in a stepwise

fashion and thus allows the length of the spacer to be adjusted

[20]. We report herein the preparation of six different

fullerene–linker–fullerene triads in which both the length of the

linker and the nature of the fullerene cage are systematically

varied, and we explore their spin-carrying and spin-tuning

capacity in reduced states using electrochemical techniques and

electron paramagnetic resonance (EPR) spectroscopy.

Results and DiscussionSynthesis of the fullerene triadsThis study aimed to vary the fullerene–fullerene separation and

the nature of the fullerene cages resulting in the preparation of

six fullerene–linker–fullerene (triad) compounds (Figure 1).

The fullerenes were functionalised via Prato reaction chemistry

forming a pyrrolidine ring across the [6.6] bond of the cage

[21]. The resulting pyrrolidine functionalised fullerenes are

known to be electrochemically stable and can be readily linked

together via the N atom to form linear and diastereomerically

pure triads. The choice of linker was determined by the target

fullerene–fullerene separation in the product, and terephthalate

and oxalate bridges were chosen as they possess similar chem-

ical properties but differ significantly in size. The distance

between the centres of the corresponding fullerene cages in the

terephthalate bridged triads (compounds 1–3) was estimated to

be 16–20 Å depending on the conformation of the molecule (see

Figure S4, Supporting Information File 1), whereas the oxalate

(compounds 4 and 6) or acetate (compound 5) bridged triads

have significantly shorter separations (12–15 Å). Function-

alised fullerenes 7 and 8, which are precursors in the synthesis

of triads 1–3, were utilised as control compounds in the electro-

chemical studies and to aid the assignment of redox processes in

the triad species.

The triads 1–3 were synthesised in five steps by functionalisa-

tion of each fullerene cage using the Prato reaction, addition of

the terephthalate spacer to the fulleropyrrolidine unit and subse-

quent coupling of two fullerene moieties [20]. The oxalate or

acetate bridged triads 4–6 were prepared using a similar

strategy. To link the two C60 or two C70 fullerene cages with

the oxalate spacer (compounds 4 and 6) a one-step procedure

was used in which the corresponding fulleropyrrolidine was

treated with an excess of oxalyl chloride in the presence of 4-di-

methylaminopyridine (DMAP) and pyridine (Scheme 1).

Oxalate bridged triads 4 and 6 were obtained in moderate yields

and displayed physical and spectroscopic properties very

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334

Figure 1: Structures of triads 1–6 and precursor molecules 7–8 used for the synthesis of the asymmetric systems. For the C70 containing compoundsonly the major (8,25) regioisomer is shown for clarity.

Scheme 1: The one-step synthetic procedure towards the oxalate-bridged fullerene triads 4 and 6.

similar to those of terephthalate bridged triads 1–3, including

good solubility in organic solvents such as CS2 and

o-dichlorobenzene (ODCB).

To link the C60 and C70 fulleropyrrolidines within asymmetric

triads with an oxalate spacer we attempted a similar stepwise

approach as reported for compounds 1–3 (Scheme 2).

Oxalic acid monobenzyl ester monochloride was prepared by

equimolar reaction of oxalyl chloride and benzyl alcohol [22]

and was reacted with [60]fulleropyrrolidine 9 to give the benzyl

ester protected compound 11 in 68% yield. Subsequent depro-

tection of 11 by CF3SO3H yielded the insoluble product 12 that

precluded characterisation by solution based methods.

However, MALDI–MS of 12 showed a molecular ion peak with

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335

Scheme 2: Attempted synthetic pathway towards the formation of the C60–C70 oxalate bridged fullerene triad allowing the coupling of the fullerenecages in a stepwise fashion.

m/z 791 and the solid state IR spectrum as a pressed disc in KBr

indicated only one signal in the carbonyl region characteristic of

the amide group (1653 cm−1) and no signal related to the

carboxylic group stretch. The desired carboxylic acid com-

pound [12] seems to be unstable under acidic conditions and

undergoes decarboxylation to form an insoluble amide com-

pound 12. This reaction under acidic conditions is character-

istic of carboxylic acids that contain an electron withdrawing

substituent in the α-position [23].

To prevent decarboxylation processes we modified the linker to

exclude the electron withdrawing amide group from the α-pos-

ition of the carboxylate functionality, while maintaining an

identical inter-fullerene separation in the asymmetric triad when

compared with the symmetric analogue (Scheme 3).

The key step in this procedure is the formation of intermediate

compound 14 via the nucleophilic substitution of the iodide

centre in benzyl 2-iodoacetate [24] (13) with [60]fulleropyrroli-

dine 9 (Scheme 3). Compound 14 is a benzyl ester of an

α-amino acid which is stable under acidic conditions. Indeed,

deprotection of 14 yielded the desired carboxylic acid 15. Com-

pound 15 also shows limited solubility, but MALDI–MS (m/z

821) and IR spectroscopy (carbonyl stretch at 1733 cm−1)

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336

Scheme 3: Synthetic pathway to the asymmetric fullerene triad 5 allowing introduction of the fullerene cages in a stepwise fashion.

Table 1: Electrochemical dataa for fullerene based compounds 1–8.

Compound E1/2 red1, V E1/2 red2, V E1/2 red3, V ΔE, Fc+/Fc

1 −1.09 (0.16) −1.47 (0.16) −2.01 (0.16) 0.172 −1.12 (0.10) −1.48 (0.08) −1.89 (0.05)

−2.02 (0.06)0.20

3 −1.12 (0.06) −1.50 (0.08) −1.89 (0.06) 0.104 −1.09 (0.16) −1.47 (0.16) −2.01 (0.18) 0.165 −1.13 (0.10) −1.49 (0.10) −1.89 (0.11)

−2.05 (0.10)0.10

6 −1.10 (0.11) −1.47 (0.10) −1.89 (0.11) 0.127 −1.11 (0.09) −1.49 (0.09) −2.02 (0.08) 0.158 −1.15 (0.06) −1.52 (0.06) −1.94 (0.06) 0.09

aPotentials (E1/2 = (Epa + Ep

c)/2) in volt are quoted to the nearest 0.01 V. All potentials are reported against the Fc+/Fc couple for 0.5 mM solutions ino-dichlorobenzene containing 0.2 M [n-Bu4N][BF4] as the supporting electrolyte. The anodic/cathodic peak separation (ΔE = Ep

a − Epc) is given in

brackets where applicable. ΔE for the Fc+/Fc couple was used as the internal standard.

confirm the assigned structure. The dicyclohexylcarbodiimide

(DCC) assisted acid–amine coupling reaction between 15 and

[70]fulleropyrrolidine 10 resulted in the formation of the

desired asymmetric triad 5, which displayed physical properties

similar to those of 1–4 and 6.

Electrochemical characterisation of fullerenetriads 1–6Compounds 1–6 were studied by cyclic voltammetry (CV) as

solutions in o-dichlorobenzene in order to investigate the

sequence of electron additions and possible electronic interac-

tions in these triads.

The cyclic voltammograms recorded for the fullerene triads 1–6

are very similar to those of their monomeric precursors 7 and 8

and do not appear to show interactions between the individual

fullerene cages. This observation is consistent with the lack of

an effective electronic communication pathway through the

linker groups within the triads (Table 1 and Figure 2).

Each triad molecule 1–6 exhibits a series of reduction processes

in a potential range between −0.3 and −2.3 V (vs Fc+/Fc), some

of which are overlapping or appear as shoulders to the main

peaks. By comparison with the cyclic voltammetry of the

precursor compounds 7 and 8 (Figure S5, Supporting Informa-

tion File 1), we suggest that this series of reductions corre-

sponds to the addition of up to 3 electrons per fullerene cage (in

total 6 electrons per molecule). No oxidation processes were

found in the range up to 1.5 V (vs Fc+/Fc). For triads 1 and 4,

containing C60 only, these potentials are similar to those of their

precursor 7. A similar correlation was noted for the C70

containing triads 3 and 6 and their precursor 8. Comparison of 1

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337

Figure 2: Cyclic voltammograms of the terephthalate bridged triads 1–3 (left) and oxalate bridged triads 4–6 (right). Data were recorded as 0.5 mMsolutions in o-dichlorobenzene containing 0.2 M [n-Bu4N][BF4] as the supporting electrolyte, at a scan rate of 100 mV.

with 3, 4 with 6 and 7 with 8 shows that the first and second

reductions of C60, in general, occur at slightly more anodic

potentials than those of C70 cages. However beyond the second

reduction, C70 is more readily reduced. For 2 and 5, each

containing a mixture of C60 and C70 fullerenes, two well

defined reduction couples are observed at E1/2 ca. −1.12 and

−1.49 V, which we assigned to an overlap of C60/C70-based

reductions. These processes are separated by an additional

process that appears as a shoulder on the first reduction in 2 and

5 (Figure 2 and Figure S6, Supporting Information File 1). A

similar feature is noted to cathodic potential of the second

reduction process. For 2 and 5 we associate these features with

the generation of a reduced C70 cage in a triad molecule, noting

a similar, although less pronounced effect in 3 and 6, each

containing two equivalent C70 cages (see Supporting Informa-

tion File 1) and an absence of these features in C60 triads, 1 and

4, and in the C70 containing dyad, 8. The origin of these

effects is unclear and may result from the nature of interaction

of the reduced triad with the electrode surface. We note

that the first and second reductions on C70 are expected to be

slightly more cathodic than those for C60 but comparing

potentials for 7 and 8, we suggest that this difference alone is

too small to explain the position of these features. We note

also that 2 and 5 are mixtures of two regioisomers of the

pyrrolidine functionalised C70 [19], in a ratio of 6:4 as deter-

mined by 1H NMR spectroscopy (see Experimental section). It

is possible that these isomers may interact with the electrode

differently.

Based on these results we can confirm that for C60–C60 triad

molecules changing the nature and the size of the bridging

group has little effect on the nature and potentials of the redox

processes. Thus, we conclude that the two C60-fullerene cages

in the triads behave independently in the ground state. These

results are consistent with other fullerene triad systems in which

intramolecular fullerene–fullerene interactions are only

observed where fullerene cages are bonded directly [25]

or bridged by a transition metal atom [26,27]. For triads

containing C70 the results are less clear where additional

electrode processes are observed. However we do not

attribute these features to intramolecular fullerene–fullerene

interactions.

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338

Figure 3: Fluid solution EPR spectra recorded at 297 K for the twoelectron reduced species of compounds 1 and 4 and the one electronreduced species of 7.

EPR spectroscopic characterisation of thefullerene triads in the reduced stateThe electron spin–spin interactions that are crucial for the appli-

cation of fullerene triads were investigated by EPR spec-

troscopy as fluid and frozen solutions at room temperature and

77 K, respectively, following electrochemical reduction. Whilst

these triads are capable of accepting multiple electrons into each

of the fullerene groups, we restrict our discussion to dianionic

species in compounds where the electrochemistry is well

defined; under these conditions each fullerene cage is reduced

by a single electron. We have evaluated the effects of varying

the inter-fullerene separation (oxalate bridge vs terephthalate

bridge) and the nature of the fullerene (C60 vs C70) on the

nature of the spin–spin coupling obtained.

The two electron reduced species of 1 and 4 (12− and 42−) and

the corresponding mono reduced species of their monomeric

precursor 7 (71−) (i.e. one electron per fullerene cage for all

species) were obtained by electrochemical reduction at −1.4 V

of 0.5 mM solutions of compound in o-dichlorobenzene

containing [n-Bu4N][BF4] as the supporting electrolyte.

Fluid solution EPR spectra of 12−, 42− and 71− (Figure 3) are

similar in g value (2.0002, 2.0001 and 2.0000, respectively) but

differ in linewidth (ΔHp−p 1.1, 1.3 and 0.8 G, respectively) and

are typical of C60 based radical anions [28], confirming that the

electrochemically introduced electrons are localised on the

fullerene cages.

The EPR spectrum of 42− in frozen solution recorded at 77 K

shows an intense central feature at g = 2.0001 (Figure 4a) indi-

cating that the majority of the molecules exist as two inde-

pendent doublet (S = 1/2) radicals and suggesting a small

singlet-triplet energy gap [29]. The intramolecular triplet birad-

ical (S = 1) of 42− is also present with a zero-field splitting para-

meter (D) of 27.8 G (Figure 4a,b). This value is in the same

range (26–29 G) as that observed for other triplet biradicals of

pyrrolidine-functionalised C60 derivatives in C60–bridge–C60

triads [15,28] and gives an average distance of 10 Å between

the unpaired electrons [16], a distance well within the range

predicted by models of 4 (Figure S4b, Supporting Information

File 1). The half-field signal corresponding to the triplet state is

not observed which is also consistent with previous reports for

fulleride based triplets [30]. The presence of an intramolecular

triplet would indicate that the distance between the two

fulleropyrrolidine units is short enough to allow through-space

interaction despite the lack of electronic conjugation between

the interacting units. In addition to the intramolecular triplet, a

set of inner features is tentatively assigned to an intermolecular

(or “powder”) triplet (D = 7.9 G) that may result from the

aggregation of 42− molecules in the frozen solution however we

do not exclude other possible assignments [31].

The frozen solution EPR spectrum of 12− displays a central

feature at g = 2.0003, consistent with that of a doublet biradical

(Figure 4c) that is flanked on each side by broad “wings” that

we assign to the presence of an intermolecular triplet and give a

maximum D value of 9 G which is similar to that observed in

the spectrum of 42−. The same “wings” around the central

feature (g 2.0000) have been observed in the EPR spectra of the

one electron reduced species of the monomer 7 (Figure S7,

Supporting Information File 1) and hence may be explained by

intermolecular interactions. Also present are small baseline

features (Figure 4d) that may represent the outer features of

either an intramolecular or intermolecular triplet. We note

similar small baseline features in the spectrum of 71−; in this

case their assignment to an intramolecular triplet must be

excluded. Hence, by changing the linker from oxalate in 42− to

terephthalate in 12− we have either reduced the interaction of

the spin centres or significantly perturbed the formation of an

intramolecular triplet biradical.

The EPR spectra of the C70 containing compounds 62− and 81−

(Figure S8, Supporting Information File 1) in fluid solution

recorded at room temperature (Figure S8a, Supporting Informa-

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339

Figure 4: Frozen solution EPR spectra of triads 42− (a) and 12− (c), prepared by two electron reduction of 4 and 1, respectively, at −1.4 V recorded at77 K in o-dichlorobenzene solution containing [n-Bu4N][BF4] as supporting electrolyte. Enlarged regions around the central feature of 42− (b) and 12−

(d) show characteristic zero field splitting parameters for the intermolecular (purple) and intramolecular (orange) triplet states.

tion File 1) and particularly in frozen solution recorded at 77 K

(Figure S8b, Supporting Information File 1) are significantly

different from those observed for the reduced C60 containing

compounds. The difference is due to the lower symmetry of the

C70 (D5h compared to Ih of the C60) which results in an

anisotropic spectrum [30]. In addition, due to this asymmetry

the spectra are significantly broader which means that any

features corresponding to the triplet biradicals will overlap with

the main central features and hence are not resolved. This

complicates the assignment of the spin states in the C70

containing compounds and hence they were not investigated

further in this study.

ConclusionWe have developed a synthetic methodology for a range of

linear, soluble fullerene triads where the nature of the fullerene

cage and the length of the bridge between the cages can be

controlled. Cyclic voltammetry measurements demonstrate the

high electron accepting capacity of these molecules, which can

accept up to six electrons reversibly, but indicate no interac-

tions between the fullerene cages in the ground state of the

triads, regardless of the nature of the fullerene (C60 or C70) or

the length of the bridge (oxalate or terephthalate). The first and

second reduction potentials of C60 and C70 in the asymmetric

triads appear to be indistinguishable, whilst the third reduction

of the two fullerene cages is observed as two separate one-elec-

tron processes with the reduction potential being slightly less

cathodic for the C70 cage. EPR spectroscopy measurements of

the two electron reduced triads reveal that the nature of the

intramolecular electron spin–spin interactions is dependent on

the length of the bridge. Specifically, the two electron reduced

oxalate bridged triad, where the fullerene cages are separated by

a minimum distance of 12 Å, can exhibit strong intramolecular

spin coupling with a D value of 27.8 G. Under the same condi-

tions the triad with the terephthalate bridge, where fullerene

cages are separated by a minimum distance of over 16 Å, does

not show similar strong intramolecular spin coupling and may

exist mainly as an independent doublet biradical. Our method-

ology enables precise control of the inter-fullerene separation

thus providing a mechanism for controlling the spin properties

of fullerene triads which is important for the future develop-

ment of molecular electronic and spintronic devices.

ExperimentalC60 (99.5%) and C70 (95%) were purchased from SES Research

and MER corporation respectively. CH2Cl2 was freshly distilled

Beilstein J. Org. Chem. 2014, 10, 332–343.

340

over CaH2 before use. All other reagents and solvents were

purchased from Aldrich and used without further purification.

Compounds 1–3 and 7–10 were synthesised according to previ-

ously reported procedure [20]. Infra-red spectra were measured

as KBr discs using a Nicolet Avatar 380 FTIR spectrometer

over the range 400–4000 cm−1. 1H and 13C NMR spectra were

obtained using Bruker DPX 300, Bruker DPX 400, Bruker

AV(III) 400 or Bruker AV(III) 500 spectrometers. Mass spec-

trometry was carried out using a Bruker microTOF spectro-

meter and a Bruker ultraFlexIII MALDI–TOF spectrometer

using trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenyl-

idene]malononitrile (DCTB) as supporting matrix. UV–vis

spectra were measured using a Lambda 25 Perkin Elmer Spec-

trometer. EPR spectra were obtained on a Bruker EMX EPR

spectrometer.

Cyclic voltammetryCyclic voltammetric studies were carried out using an Autolab

PGSTAT20 potentiostat, using a three-electrode arrangement in

a single compartment cell. A glassy carbon working electrode, a

Pt wire secondary electrode and a saturated calomel reference

electrode (chemically isolated from the test solution via a bridge

tube containing electrolyte solution and fitted with a porous

Vycor frit) were used in the cell. Experiments were performed

under an atmosphere of argon and in anhydrous solvents.

Sample solutions were prepared under an atmosphere of argon

using Schlenk line techniques and consisted of a 0.2 M

[n-Bu4N][BF4] solution as the supporting electrolyte and a

0.5 mM solution of the test compound. Redox potentials were

referenced vs the Fc+/Fc couple, which was used as an internal

standard. Compensation for internal resistance was not applied.

Bulk electrolysisBulk electrolysis experiments at a controlled potential were

carried out using a two-compartment cell. A Pt/Rh gauze basket

working electrode was separated from a wound Pt/Rh gauze

secondary electrode by a glass frit. A saturated calomel elec-

trode was bridged to the test solution through a Vycor frit that

was orientated at the centre of the working electrode. The

working electrode compartment was fitted with a magnetic

stirrer bar and the test solution was stirred rapidly during elec-

trolysis. Each solution contained [n-Bu4N][BF4] (0.2 M) as the

supporting electrolyte and the compound under investigation

(5 mL, 0.5 mM) and were prepared using Schlenk line tech-

niques.

Synthesis of the fullerene triads 4–6Synthesis of 4[60]Fulleropyrrolidine 9 (35 mg, 0.045 mmol) was suspended in

freshly distilled CH2Cl2 (35 mL) with 4-dimethylaminopyridine

(17 mg, 0.139 mmol) and pyridine (0.35 mL), and the reaction

mixture was stirred for 10 min. Oxalyl chloride (0.1 mL, 1.16

mmol) was then added, and the reaction mixture stirred at room

temperature for 2 h. On removal of solvent, the resulting residue

was dissolved in CS2 (40 mL) by sonication, and any undis-

solved material removed by filtration. The filtrate was concen-

trated and purified by column chromatography (silica gel,

o-dichlorobenzene/isopropyl alcohol 99.5:0.5) to give the prod-

uct which was washed with MeOH (30 mL), petroleum ether

40–60 (30 mL) and diethyl ether (30 mL). The resultant solid

was dried under vacuum to give the product (16 mg, 44%) as a

brown solid. 1H NMR (400 MHz, 297 K, CS2/CDCl3, δ, ppm)

5.79 (s, 4H, CH2), 5.69 (s, 4H, CH2); 13C NMR (125 MHz,

297 K, CS2/CDCl3, δ, ppm) 160.97 (CO), 153.09, 152.74,

147.47, 146.46, 146.44, 146.26, 146.24, 145.80, 145.72, 145.46,

145.43, 145.40, 145.38, 144.59, 144.46, 143.21, 142.83, 142.80,

142.30, 142.23, 142.21, 142.16, 142.07, 142.05, 140.50, 140.40,

135.97, 135.76, 133.02, 130.61, 128.44, 127.69 (sp2 carbons, 31

environments), 70.85, 69.31, 60.22, 57.07 (sp3 carbons, 4 envi-

ronments); MALDI–TOF MS (DCTB/MeCN, m/z): 1580.2

(M−); IR (KBr, ν, cm−1): 2960 (w), 2922 (w), 1645 (s), 1453

(w), 1328 (m), 1186 (m), 746 (m), 526 (s); UV–vis (CS2): λmax

(ε × 10−3/dm3 mol−1 cm−1): 703 (0.750), 433 (8.526).

Synthesis of 6[70]Fulleropyrrolidine 10 (20 mg, 0.023 mmol) was suspended

in CH2Cl2 (18 mL), and DMAP (8.7 mg, 0.071 mmol) and

pyridine (0.2 mL) were added. The reaction mixture was stirred

for 10 min and oxalyl chloride (0.055 mL, 0.64 mmol) was

added. The reaction mixture was stirred at room temperature for

2 h, and the solvent was then removed. The resulting residue

dissolved in CS2 (40 mL) by sonication, and the undissolved

material removed by filtration. The filtrate was concentrated

and purified by column chromatography (silica gel,

o-dichlorobenzene/isopropyl alcohol 99.5:0.5) to give the prod-

uct which was washed with MeOH (30 mL) and petroleum

ether 40–60 (30 mL). The resultant material was dried under

vacuum to give the product (9 mg, 22%) as a brown solid.1H NMR (400 MHz, 297 K, CS2/CDCl3, δ, ppm) 5.25–4.36 (m,

8H, CH2); 13C NMR (125 MHz, 297 K, CS2/CDCl3, δ, ppm)

168.90 (C=O), 167.43 (C=O), 156.55, 156.13, 155.02, 154.96,

154.11, 152.17, 151.56, 151.42, 151.35, 151.07, 151.01, 151.00,

150.96, 150.94, 150.78, 150.71, 150.41, 150.37, 149.93, 149.89,

149.85, 149.79, 149.47, 149.40, 149.36, 149.31, 149.26, 149.11,

149.07, 149.03, 148.79, 148.45, 148.10, 148.06, 147.88, 147.46,

147.21, 147.10, 147.08, 147.05, 147.00, 146.96, 146.93, 146.64,

146.62, 146.57, 145.84, 145.71, 145.53, 145.41, 145.00, 144.90,

144.84, 144.50, 144.48, 144.23, 144.20, 144.16, 144.12, 144.08,

143.48, 143.43, 143.32, 143.26, 143.18, 143.10, 141.43, 140.32,

140.30, 140.27, 137.28, 133.76, 133.72, 133.68, 132.41, 132.13,

131.38, 131.28, 131.25, 128.40 (sp2 carbons, 80 environments),

71.84, 69.68, 68.74, 64.10, 62.67, 62.63 (sp3 carbons, 6 envi-

Beilstein J. Org. Chem. 2014, 10, 332–343.

341

ronments); MALDI–TOF MS (DCTB/MeCN, m/z): 1820.5; IR

(KBr, ν, cm−1): 2932 (m), 2363 (s), 1663 (s, C=O), 1435 (m),

669 (s); UV–vis (CS2): λmax (ε × 10−3/dm3 mol−1 cm−1): 693

(3.74), 556 (20.32), 476 (39.28), 462 (39.60), 411 (48.49).

Synthesis of 5Compound 15 (16 mg, 0.018 mmol), [70]fulleropyrrolidine 10

(16 mg, 0.019 mmol) and dicyclohexylcarbodiimide (3.9 mg,

0.019 mmol) were suspended in anhydrous o-dichlorobenzene

(2.7 mL) and stirred at room temperature under an Ar atmos-

phere for 17 h. The reaction mixture was purified by column

chromatography (silica gel, o-dichlorobenzene) to afford the

product which was washed with MeOH (30 mL) and petroleum

ether (30 mL) to give the product (9 mg, 27%) as a dark brown

solid. Isomer a (see Supporting Information File 1): 1H NMR

(400 MHz, 297 K, CS2/CDCl3, δ, ppm) 5.47 (s, 4H, CH2), 5.28

(m, 2H, CH2), 4.77 (s, 2H, CH2), 4.66 (s, 2H, CH2); Isomer b

(see Supporting Information File 1): 1H NMR (400 MHz,

297 K, CS2/CDCl3, δ, ppm) 5.63 (s, 1H), 4.97 (m, 1H), 4.88 (s,

1H), 4.61 (m, 2H), 4.27 (s, 2H), 4.16 (s, 1H), 4.11 (m, 2H);13C NMR (125 MHz, 297 K, CS2/CDCl3, δ, ppm) 165.69

(C=O), 155.45, 155.16, 155.07, 154.23, 153.30, 153.19, 151.74,

151.38, 151.21, 151.19, 150.92, 150.79, 150.74, 150.66, 149.91,

149.89, 149.81, 149.36, 149.35, 149.32, 149.24, 149.09, 148.78,

148.44, 148.13, 147.61, 147.41, 147.21, 146.94, 146.36, 146.18,

146.15, 145.94, 145.89, 145.57, 145.40, 145.31, 144.64, 144.60,

143.45, 143.22, 143.07, 142.78, 142.18, 142.03, 140.60, 140.38,

139.15, 137.17, 134.25, 133.75, 132.92 (sp2 carbons, 52 envi-

ronments), 74.22, 73.26, 70.50, 68.18, 68.14, 64.51, 63.11 (sp3

carbons, 7 environments); MALDI–TOF MS (DCTB/MeCN,

m/z): 1686.3; IR (KBr, ν, cm−1): 2926 (s), 2365 (s), 1674 (s,

C=O), 1433 (m), 1250 (m), 1182 (m), 1119 (m), 671 (w), 527

(m); UV–vis (CS2): λmax (ε × 10−3/dm3 mol−1 cm−1): 693

(2.51), 556 (11.83), 479 (22.00), 456 (22.91), 430 (22.48), 410

(30.38).

Oxalic acid monobenzyl ester monochloride[22]Oxalyl chloride (1 mL) was cooled to 0 °C, and anhydrous

benzyl alcohol (1.4 mL) added dropwise over 15 min. After the

addition of the alcohol was completed, the reaction mixture was

warmed up to room temperature and stirred for 1.5 h. The

resulting mixture was analysed by 1H and 13C NMR spec-

troscopy and found to be a mixture of the oxalic acid

monobenzyl ester monochloride and the dibenzyl oxalate in a

5:1 molar ratio. The mixture was used in the next step immedi-

ately without further purification. 1H NMR (300 MHz, 297 K,

CDCl3, δ, ppm) 7.47–7.42 (m, 5H), 7.42–7.38 (m, 1.75 H), 5.38

(s, 2H), 5.32 (s, 0.72 H); 13C NMR (75 MHz, 297 K, CDCl3, δ,

ppm) 160.95, 157.57, 155.53, 134.18, 133.33, 129.35, 128.91,

128.78, 128.74, 70.38, 68.63.

[60]Fulleropyrrolidine oxalate benzyl ester 11[60]Fulleropyrrolidine 9 (70 mg, 0.092 mmol) was suspended in

freshly distilled CH2Cl2 and DMAP (50 mg, 0.41 mmol) and

pyridine (0.3 mL) added. The mixture was stirred for 10 min at

room temperature, and oxalyc acid monobenzyl ester mono-

chloride (100 mg, 0.50 mmol) was added, and the mixture left

to stir at room temperature for 2 h. The solvent was removed,

the resulting residue dissolved in CS2 (10 mL) and filtered to

remove insoluble materials. The filrate was then concentrated

and purified by column chromatography (silica gel, toluene) to

afford the product which was washed with MeOH (40 mL) and

petroleum ether (40 mL) and dried in vacuum to give com-

pound 11 as black powder (58 mg, 68%). 1H NMR (400 MHz,

297 K, CDCl3/CS2, δ, ppm) 7.47 (d, J = 6.3 Hz, 2H), 7.35 (m,

3H), 5.51 (s, 2H), 5.45 (s, 4H); 13C NMR (125 MHz, 297 K,

CDCl3/CS2, δ, ppm) 160.85 (C=O), 157.49 (C=O), 153.01,

152.57, 147.50, 147.44, 146.49, 146.44, 146.26, 146.24, 145.80,

145.76, 145.61, 145.46, 145.45, 145.43, 145.31, 144.62, 144.51,

143.22, 142.83, 142.82, 142.25, 142.18, 142.10, 142.03, 140.46,

140.32, 137.51, 136.25, 135.94, 134.60, 129.15, 128.99, 128.96,

128.66, 128.44, 128.42, 125.53 (sp2 carbons, 37 environments),

70.67, 69.09, 67.99, 59.35, 57.16 (sp3 carbons, 5

environments); MALDI–TOF MS (DCTB/MeCN, m/z): 924.1

(M−); IR (KBr, ν, cm−1): 2924 (m), 2362 (m), 1718 (s, C=O),

1671 (s, C=O), 1438 (m), 1125 (s), 527 (s).

Synthesis of 12Compound 11 (5 mg) was dissolved in freshly distilled CH2Cl2

(5 mL), and CF3SO3H (0.05 mL) added. The resulting mixture

was stirred for 1 h at room temperature after which the solvent

was removed under vacuum and the resultant brown solid was

suspended in diethyl ether (10 mL). The precipitate was sep-

arated by centrifugation, the ether removed by decantation, and

this procedure was repeated three times. The resultant brown

solid was dried under vacuum to give the product, 12 (4 mg,

95%) as a brown solid. MALDI–TOF MS (DCTB/MeCN,

m/z): 791.2 (M−); IR (KBr, ν, cm−1): 3446 (s, NH), 2964 (m),

2360 (m), 1636 (s, C=O), 1507 (m), 1384 (s), 1216 (m), 527

(m).

Synthesis of 13 [24]NaI (5.3 g, 34 mmol) was suspended in acetone (15 mL) and

heated under reflux for 5 min. The mixture was cooled to room

temperature, and benzyl bromoacetate (1 mL, 6.3 mmol) added.

The reaction mixture was stirred at room temperature for 2 h

and the solvent removed under vacuum. The resulting mixture

was partitioned between water (20 mL) and ethyl acetate

(10 mL). The organic fraction was separated, washed with a

saturated solution of Na2S2O3 (2 × 10 mL) followed by brine

(10 mL) and dried over Na2SO4. The solvent was removed to

give the product (1.57 g, 90%) as a yellow oil; 1H NMR

Beilstein J. Org. Chem. 2014, 10, 332–343.

342

(300 MHz, 297 K, CDCl3, δ, ppm) 7.40 (s, 5H), 5.20 (s, 2H),

3.76 (s, 2H); 13C NMR (75 MHz, 297 K, CDCl3, δ, ppm)

168.60 (C=O), 135.14, 128.65, 128.54, 128.33, 67.81

(-CH2-O-), -5.51 (-CH2I); ESIMS (MeOH, m/z): 298.95

(M + Na)+.

Synthesis of 14To a solution of [60]fulleropyrrolidine 9 (110 mg, 0.144 mmol)

in dry DMF (30 mL) benzyliodoacetate (150 mg) was added,

and the resulting mixture heated to 100 °C for 1 h. The solvent

was then removed under vacuum, and the resulting solid puri-

fied by column chromatography (silica gel, eluted with CS2,

followed by CS2/toluene 1:1 v/v). The product was washed with

MeOH (40 mL) and the resultant solid was dried in vacuum to

give compound 14 as a black powder (80 mg, 61%). 1H NMR

(400 MHz, 297 K, CDCl3/CS2, δ, ppm) 7.49 (d, J = 6.8 Hz,

2H), 7.42 (m, 3H), 5.37 (s, 2H), 4.68 (s, 2H), 4.09 (s, 2H);13C NMR (125 MHz, 297 K, CDCl3/CS2, δ, ppm) 169.31

(C=O), 154.61, 147.37, 146.33, 146.14, 146.03, 145.74, 145.56,

145.36, 144.62, 143.18, 142.72, 142.28, 142.16, 141.98, 140.26,

136.39, 135.64, 135.22, 128.81, 128.62, 128.53, 128.37 (sp2

carbons, 22 environments), 70.58, 67.05, 66.78, 54.84 (sp3

carbons, 4 environments); MALDI–TOF MS (DCTB/MeCN

m/z): 911.5 (M−); IR (KBr, ν, cm−1): 2962 (w), 2359 (w), 1736

(s, C=O), 1393 (m), 1344 (m), 1095 (s), 737 (m), 527 (s).

Synthesis of 15To a solution of 14 (5 mg, 0.0055 mmol) in dry CH2Cl2 (5 mL)

CF3SO3H (0.05 mL) was added and the mixture was stirred for

2 h at room temperature. The solvent was removed under

vacuum and the resultant brown solid suspended in diethyl ether

(10 mL). The precipitate was separated by centrifugation, the

ether removed, and this procedure was repeated three times.

The resultant brown solid was dried in vacuum to give the prod-

uct, 15 (4.2 mg, 90%); MALDI–TOF MS (DCTB/MeCN,

m/z): 821.2 (M−); IR (KBr, ν, cm−1): 3446 (s, OH), 2957 (w),

2361 (w), 1732 (m, C=O), 1483 (m), 1170 (m), 746 (m),

527 (s).

Supporting InformationSupporting Information File 1Additional spectra.

[http://www.beilstein-journals.org/bjoc/content/

supplementary/1860-5397-10-31-S1.pdf]

AcknowledgementsWe thank the European Research Council, the EPSRC, the

Royal Society and the University of Nottingham for financial

support of this work.

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